15: The Chemical Senses

Question 1

Define olfaction and gustation.

Answer 1

Olfaction: sense of smell.

Gustation: sense of taste.

Question 2

One role of olfaction and gustation is to act as “gate-keepers.” What does this mean?

Answer 2

Sense the properties of chemicals that are just about to be assimilated into the body and whether they are good or bad.

Question 3

“Wear and tear” on chemical receptors is caused by what?

Answer 3

Dust/dirt, viruses and bacteria, damaging chemicals.

Question 4

Constant renewal of receptors is called what?

Answer 4

Neurogenesis.

Question 5

What is the life cycle (birth – development – death - replacement) span of olfactory receptors and taste receptors?

Answer 5

Olfactory: 5-7 weeks

Taste: 1-2 weeks

Question 6

Aside from perceiving odors, what are other functions for the olfactory system relating to reproduction?

Answer 6

Influence reproductive physiology in conspecifics.

Influence estrus cycles.

Regulate onset of puberty.

Prevent implantation of fertilized embryos.

Signal receptivity for mating.

Question 7

What is a measure regarded as a technical advancement for olfaction research?

Answer 7

Olfactory stimuli delivered by evaporation from cotton balls vs. controlled delivery through a specialized olfactometer.

Question 8

Define macrosmatic and microsmatic. What are humans and what are dogs, in this case?

Answer 8

Macrosmatic: possessing a keen sense of smell that is important to behaviour.

Microsmatic: possessing a less keen sense of smell that is less crucial for survival.

Humans microsmatic, dogs macrosmatic.

Question 9

Define detection threshold with regards to smell. What can affect it?

Answer 9

Lowest concentration (in parts per billion) at which an odorant can be detected.

Can be affected by nature of testing.

Question 10

Rats are _____ times more sensitive to odors than humans. Dogs are _____ times more sensitive to odors than humans.

Answer 10

8-50; 300-10,000.

Question 11

Individual olfactory receptors have the same sensitivity across species. What is it?

Answer 11

Can detect just 1 molecule of an odorant.

Question 12

Interspecies differences in sensitivity arise from what? What is the difference between humans and dogs?

Answer 12

The number of receptors in the olfactory epithelium.

Humans: ~1-10 million receptors (surface area ~10cm2).
Dogs: ~1 billion receptors (surface area ~170cm2).

Question 13

Detecting an odor is not the same as _____ the odor. Concentrations required for _____ are as much as 3X higher than detection threshold.

Answer 13

Identifying; identification threshold.

Question 14

With no clues, we can only identify an odor about _____ of the time. If given a few names candidate smells (like a multiple choice exam), we can identify an odor about _____ of the time.

Answer 14

50%; 98%.

Question 15

We can still describe the qualities for odors that we do not instantly recognize. Why is this?

Answer 15

Lack of recognition is not related to a deficiency in sensitivity, rather an inability to retrieve the odor’s name from memory.

Question 16

List the four steps of the olfactory pathway.

Answer 16

Odorants pass over the olfactory mucosa.

Activate olfactory receptor neurons.

Sensory neurons send organized inputs to glomeruli in the olfactory bulb

Olfactory bulb projects to the primary olfactory cortex (AKA piriform cortex) and other areas.

Question 17

Cells in the olfactory mucosa exude a thin coat of mucus that is fully replaced every 10 minutes. This mucus contains what two things?

Answer 17

Antibodies: protect organism from viral/bacterial invaders.

Odorant binding proteins: concentrate odorant in the mucus.

Question 18

Olfactory receptor neurons have a single _____ with several cilia that extend into the _____. It is likely that single olfactory receptor neurons express _____ of odorant receptor.

Answer 18

Long dendrite; olfactory mucosa; one type.

Question 19

Each odorant receptor has 7 transmembrane domains. How many different receptors are there in mice vs. humans?

Answer 19

Mouse: 1000

Human: 350

Question 20

What is the process of olfactory transduction?

Answer 20

Odorant binds receptor → G-protein is activated → adenylyl cyclase activated → formation of cAMP → cAMP opens cation channel → depolarization.

Question 21

Humans have ~350 different types of odorant receptors, but can discriminate ~100,000 odors. What does this suggest?

Answer 21

We use a combinatorial code for odor.

Question 22

Instead of one receptor recognizing one molecule, a receptor responds to multiple molecules, but odor identity is coded by the population activity, called the _____.

Answer 22

Recognition profile.

Question 23

What is a problem with the simplicity of the recognition profile?

Answer 23

Some molecules with similar structures smell different from each other. Some molecules with different structures smell the same.

Question 24

Olfactory receptor neurons send highly organized projections through the _____ to _____ in the olfactory bulb.

Answer 24

Cribiform plate; glomeruli.

Question 25

Axons of olfactory receptor neurons do not converge into a nerve bundle, but instead send diffuse projections through what?

Answer 25

The skull.

Question 26

Head trauma resulting in shearing of sensory neuron’s axons results in the loss of smell. What is the term for this?

Answer 26

Anosmia.

Question 27

There is about 10,000 of each type of olfactory receptor neuron in humans. This means we have how many neurons total?

Answer 27

350 x 10,000 = 3.5 million

Question 28

Each population of olfactory receptor neurons that contain one type of olfactory receptor end their input to _____ glomeruli.

Answer 28

One.

Question 29

What did an optical imaging study in the olfactory bulb show?

Answer 29

Imaged hemoglobin usage in the rat olfactory bulb. As length of carbon chain increases, the activated area moves more anterior (chemotopic map). Carboxylic acids activate a different part of olfactory bulb than aliphatic alcohols.

Question 30

There is evidence that odors are mapped onto the olfactory bulb based on their _____. Unknown how this mapping related to _____.

Answer 30

Chemical properties; smell perception.

Question 31

What is the relationship between the olfactory bulb and the primary olfactory (piriform) cortex?

Answer 31

Olfactory bulb makes a direct cortical projection to it.

Question 32

The piriform cortex sends projections to what two areas? Direct projections to what other two areas?

Answer 32

Sends projections to the thalamus, then the secondary olfactory cortex in the orbital frontal cortex.

Sends direct projections to the amygdala and entorhinal cortex.

Question 33

In the _____, areas representing different odors are diffuse, and tend to _____ with cells coding for other odors.

Answer 33

Cortex; overlap.

Question 34

Overlap may be related to what?

Answer 34

Perception of complex odors.

Question 35

Coffee gives off >100 molecules, how do we process that?

Answer 35

Organize inputs into ‘odor objects.’

Question 36

Odor objects may be represented by _____ in neural networks.

Answer 36

Hebbian-style connections.